A deceased man's penis, erect by way of Rigor Mortis.
A male corpse's fully extended genital member.
Sometimes used with other terms such as: "Mother fucking corpdick" which would refer to the necrophiliac sexual intercourse between a mother and the aforementioned recently deceased man.
It is occasionally alternatively spelled as "Corpdic"

Bill: You are such a corpdick!
Ted: Wtf does that even mean?
Bill: You're a dead man's hardened cock, idiot.

Greg: That dude doesn't even seem to breathe when he sleeps. This one time his girlfriend thought he was dead, but she was confused since he had a raging boner.
Phil: So she thought he had corpdick?
Greg: Yeah, pretty much.